Supercooled emulsion

Another way to induce precrystallization is by recirculating part of the supercooled emulsion back into the fresh product flow. TTie effects are similar to those of a precrystallization by the process described. 

A large number of studies have dealt with the behavior of water below 0°C (e.g., supercooling of water-in-oil emulsions) and determinations of free and bound water, around OX [20-25]. The crystalUzation enthalpy of water depends on temperature (see Ref. 19), which may be important in supercooled emulsions. Moreover, the difference in the specific heats of ice [2.05 J/(g K)] and water [4.18 J/(g K)] may introduce some error. 

Bunjes H., Siekmann B., and Westesen K., Emulsions of supercooled melts a novel drug delivery system, in Submicron Emulsions in Drug Targeting and Delivery, Benita S., ed., Harwood Academic Publishers, Amsterdam, 1998, 175. 

One drawback with NMR is that the liquid signal from water in o/w emulsions has to be subtracted to obtain the true SFC. This can be done by analyzing emulsions and fat blends with no tendency to supercooling and making calibration curves. Another possibility is to measure reference samples without fat and calculate the true SFC by subtracting the signal from the water and water-soluble components9. 

In the manufacture of margarine, the emulsion is processed in a scraped-surface heat exchanger that must supercool the melted fat quickly in order to form as many crystal nuclei as possible (11). 

Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) is a technique with the potential to determine the relative amounts of free and emulsified water. The freezing, or more correctly, the supercooling behavior of emulsified water is very different from that of free water, so the amount of free versus emulsified water in a sample can be characterized. This parameter is important in the characterization of produced fluids and interface emulsions in which water might exist simultaneously as both continuous and emulsified phases. 

Figure 30. Freezing behavior of an emulsion characterized by differential scanning calorimetry. The free water will freeze at approximately 273 K. Emulsified water will supercool and freeze at lower temperatures, depending upon size distribution. The smallest droplets freeze last because of the smaller volume, and so fewer nucleation sites are available for ice crystal formation and water freezing. The different freezing behavior of free versus emulsified water gives this technique the potential to quantify the relative proportions of these two types of water. (Reproduced with permission from reference 114.

The same apparatus was used to measure the kinetics of emulsion crystallization under shear. McClements and co-workers (20) showed that supercooled liquid n-hexadecane droplets crystallize more rapidly when a population of solid n-hexa-decane droplets are present. They hypothesized that a collision between a solid and liquid droplet could be sufficient to act as a nucleation event in the liquid. The frequency of collisions increases with the intensity of applied shear field, and hence shearing should increase the crystallization rate. A 50 50 mixture of solid and liquid n-hexadecane emulsion droplets was stored at 6 -0.01 °C in a water bath (i.e., between the melting points and freezing points of emulsified n-hexadecane). A constant shear rate (0-200 s ) was applied to the emulsion in the shear cell, and ultrasonic velocities were determined as a function of time. The change in speed of sound was used to calculate the percentage solids in the system (Fig. 7). Surprisingly, there was no clear effect of increased shear rate. This could either be because increase in collision rate was relatively modest for the small particles used (in the order of 30% at the fastest rate) or because the time the interacting droplets remain in proximity is not affected by the applied shear. 

McClements, D.J., S.W. Han, and S.R. Dungan, Interdroplet Heterogeneous Nucleation of Supercooled Liquid Droplets by Solid Droplets in Oil-in-Water Emulsions, Ibid. 71 1385-1389(1994). 

The supercooling is also observed with protein (BSA, casein, lactoglobulin) in addition to the aqneous phase-Cjg system, bnt the freezing point of hexadecane increases to 18.2°C. This indicates that the crystallization of the hexadecane is affected by the presence of surface-active molecules. The supercooling will have extensive dependence on various interfaces, such as emulsions, oil recovery, and immunological systems. The adsorption of proteins from aqueous solutions on snrfaces has been studied by neutron reflection. ... 

When water is finely dispersed as an aerosol, an emulsion, or as small clusters in polymeric host media, its thermal behavior can deviate significantly from that exhibited by bulk water. The reasons for these deviations are examined, and a statistical-mechanical approach for their study is proposed. A rough estimate is obtained for the depression of the temperature of maximum mean density for small spherical droplets. An explanation is advanced (in terms of specific structural fluctuations) for the singular behavior of strongly supercooled water that has been observed in emulsions near -40 by Angell and collaborators. 

Figure 3 Effect of heating (open points)-coolmg (filled points) cycle on ultrasonic velocity in a 20% hexadecane-in-water emulsion (adapted from Ref. 23). The speed of sound in the emulsion decreases with temperature and there is an abrupt change corresponding to the phase transi tion in the droplet oil. Supercooling of the liquid oil is responsible for the hysteresis loop observed.

An effective way of supercooling a liquid is to subdivide the sample into small droplets. If the number of droplets is large compared to the number of nucleation-triggering impurities, a large fraction of the droplets can be extensively supercooled [80,87]. Cloud chambers [88-91], and emulsions in an immiscible host liquid with a lower freezing point [92,93] utilize this principle, and have been extensively applied to the study of supercooled liquids. Typical droplet sizes are 1 jxm in cloud chambers, and 10 im in emulsions [94]. 

From these evidences, I believed the discontinuity of the LDA HDA transition (the arrow-1 of Fig. 12b) and the thermodynamic connection between HDA and liquid water (the arrow-2 of Fig. 12b). This was the reason why I believed the LLCP hypothesis shown by the broken line-3 of Fig. 12b. The hypothesis seemed the simplest explanation for these experimental results. However, these evidences did not offer decisive proofs and, therefore, confusion of explanations existed. Although we could not affirm the discontinuity between LDA and HDA, and although some researchers suspected that HDA, the collapsed ice Ij, might be microcrystalline, I began the experiments of the supercooled liquid water in order to search for the hypothetical LLCP (or paradoxically for a clear disproof of LLCP). Here, I always used the emulsified sample to hinder both the crystallization of the liquid and the crystal-crystal transition. The surfactant of the emulsion hardly dissolved in water, and the thermodynamic data of the emulsified water were practically the same as those of pure water in the region of overlap. 

Another aspect of the properties of the inverse micellar water pool is the ability referred to by Balny et al. [120] to circumvent freezing at zero degrees. This supercooling is a feature observed also in water-in-oil emulsions, and opens up... 

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